The control system of the present invention is designed to be utilized in a forehearth for thermally conditioning molten glass received from a glass melting
furnace and dispensed for subsequent treatment in a glassware forming machine. A glass forehearth typically includes an elongated refractory channel which is divided into a series of sections or temperature control zones for selectively reducing the temperature of the glass and achieving thermal homogeneity to provide a molten glass material of a suitable viscosity for the subsequent forming process. Each temperature control zone of the glass forehearth typically incorporates a plurality of burners, electrodes or other heat input elements spaced along the refractory channel sidewalls to replace heat lost to the refractory channel sidewalls and, in many instances, one or more cooling input elements to selectively remove heat from selected portions of the flowing molten glass.
Problems associated with the operation and control of the forehearth and with the conditioning of molten glass are common due to the limitations inherent in the design of the forehearth and the glass conditioning process. For example, if the forehearth's entrance is near the throat of a melter and the melter is susceptible to upsets due to, frequent tonnage changes, the upsets enter the forehearth almost unchanged thereby affecting the glass exit temperature and thermal homogeneity. Moreover, if the forehearth is pulled harder than designed or if the forehearth is required to remove more heat than designed, the forehearth will be susceptible to external influences such as fluctuations in air temperature and humidity and changing air flow patterns around the forehearth. It will be appreciated that fluctuations in ambient air conditions not only affects the thermal dynamics of the forehearth, but also affects the cooling air which is used as part of the temperature control system.
In addition to external temperature influences on forehearth control, forehearth operation is affected by changes in molten glass level variations and molten glass flow characteristics. During flow of the molten glass in the forehearth the molten glass is naturally cooled on the sides and bottom where heat is conducted through the insulation creating a temperature differential such that those types of molten glass which become more viscous at lower temperatures exhibit an increased flow rate between the top center glass and the side and bottom glass.
It will be appreciated that with the advent of new forming processes such as narrow neck press and blow and light weighting more precise forehearth temperature control is required to achieve and maintain the resultant desired glass production properties.
Accordingly, when operating a forehearth it is an object that the forehearth and forehearth control system maintain the exit glass temperature at a stable temperature and thermal homogeneity regardless of outside influences. In addition to stabilizing the exit glass temperature and thermal homogeneity, it is an object that the forehearth temperature control system correct the exit glass temperature in minimum time and with minimum upset when a new forming temperature is desired. Finally, it is an object that the forehearth control system should facilitate in setting up and stabilizing the forehearth during a job change and bring the exit temperature and thermal homogeneity to the desired values in minimum time and maintain them as the forehearth reaches its new equilibrium point under the new tonnage.
While current control systems have been able to achieve some of the objectives to some degree, current control systems have not been able to achieve most or all of the objectives consistency. In view of the foregoing there is still a need for an improved forehearth control system capable of precise control of forehearth temperature and thermal homogeneity over a plurality of zones with greater economy of design.